(1) Field of the Invention
The invention relates to a method for increasing strength and/or reducing strength variations in multi-layer wood, plywood, or the like sandwich material having a multiplicity of laminated wood veneers, said method comprising: measuring individual wood veneers for dry substance density, as well as for longitudinal and lateral distributions thereof, with high-frequency electromagnetic resonance; sorting wood veneers on the basis of said dry substance density for at least two different density categories, the dry substance density in a first density category being higher than in a second density category; laying wood veneers on top of each other for building multi-layer wood, plywood, and the like sandwich material consisting of a multiplicity of laminated wood veneers, such that veneers included in the first density category are placed as surface veneers, and veneers included in the second density category are placed as inner or middle veneers.
(2) Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
U.S. Pat. No. 5,524,771 discloses a method of the above type, which has enabled a substantial increase in strength and at the same time a substantial reduction in strength variations in multi-layer wood, plywood, and the like materials consisting of a multiplicity of laminated wood veneers. A method and a sensor based on high-frequency electromagnetic resonance have been described in U.S. Pat. No. 4,739,249. It has been concluded, however, that although high-frequency electromagnetic resonance enables a high-speed and reliable measurement regarding dry substance density which represents strength and internal strength variations of wood veneers, and regarding longitudinal and lateral distributions of dry substance density, even that is not capable of detecting all flaws appearing in wood veneers, or the reason for a detected anomaly in density is not unambiguous, and thus cannot be applied as a basis for sorting. For example, knots contained in wood veneers, which also have an influence on the strength of a wood veneer, cannot generally be visualized by means of high-frequency electromagnetic resonance. Of course, the wood veneers containing knots can be detected and sorted visually, but such a manual inspection leads to a slower sorting of wood veneers and, especially when conducted in a hurry, is inaccurate and unreliable. In addition, it requires a highly skilled person to do it, prohibits automated sorting, and thus increases costs of sorting.
Another possibility of detecting knots is to apply ultrasonic measuring, a method and apparatus for this being described in the article: James D. Logan, “Machine Sorting of Wood Veneer for Structural LVL Applications”—34th International Particleboard/Composite Materials Symposium Proceedings, Apr. 4-6, 2000. However, an ultrasound apparatus may be an expensive investment and there are downsides, including first of all that a good contact must be established between an ultrasonic sensor and a wood veneer, which is very difficult when dealing with a dried and waving wood veneer, and that the temperature of a wood veneer has an impact on the measuring result. Eventual arrangements, in an attempt to eliminate some of the effects of temperature, increase costs even further. Secondly, it must be noted that ultrasonic measuring comprises at least one-time reading, i.e. a process which is indeed considerably slower than measuring performed with high-frequency electromagnetic resonance, and consequently, in an eventual combination of the above, the ultrasonic measuring constitutes a speed limiting factor and, thus, the sorting is not as fast and effective as it otherwise could be. A further drawback in ultrasonic measuring is the breakdown of wood veneers resulting from mechanical contact between a sensor applied therein and a wood veneer, whereby such veneers become useless.